1. Field of the Invention
The present invention relates to nozzles and, more particularly, to a nozzle which minimizes the roughness along the edges of the column of water emitted by the nozzle.
2. Description of the Prior Art
The use of ultrasound to nondestructively inspect composite laminate parts for porosity, delaminations, and defects in bonding, is well known. Testing with ultrasound requires that a coupling medium, typically water, be used to transmit the ultrasonic waves between the transducer and the test object. Early apparatus required the test object to be submerged in water, while more recent devices use a nozzle to produce a column of water that extends between the transducer and the test object, and lies colinear with the ultrasonic waves.
A problem inherent in the use of a column of water to couple a pulse echo transducer with a test object is noise. Noise occurs when roughness along the surface of the water column causes multiple reflections of ultrasonic waves back to the transducer, resulting in spurious signals that interfere with the signals reflected from the test object that are used to analyze the composite laminate of the test object.
The spurious reflections also cause the decay of the outgoing wave at a higher rate that would otherwise be the case. The resultant loss in signal strength further reduces the signal to noise ratio when ultrasound is used in the pulse echo mode, as well as in the through transmission mode.
To retain an acceptable signal to noise ratio with the nozzles of the prior art, the emitting transducer must be maintained at a distance close enough to the test object to ensure that the edges of the column remain smooth before the column impinges on the test surface. When the surface undulates, this constraint requires incessant in and out movement of the nozzle normal to the surface of the test object.
Noise is also caused when the water column impinges on the test object and creates droplets that fall back into the column and create turbulence. As the size and frequency of such water droplets is proportional to the roughness of the surface of the water column, the noise from this effect increases with the column's surface roughness.
Efforts have been undertaken to improve the signal to noise ratio of ultrasonic testing apparatus by reducing the surface roughness of the coupling water column. Although it has been found that surface roughness is proportional to the spiral component of the velocity vector of the fluid in the column, efforts to design a nozzle to minimize this vector have not met with success.
Furthermore, it has been found that devices which force the water to flow through axial orifices in attempting to minimize the spiral component of the water flow, typically trap an air bubble adjacent to transducer face from which the ultrasonic waves are emitted. The air bubble causes reflection of the emitted ultrasonic waves, resulting in a further degradation of the signal to noise ratio of the transducer.